1. Field of the Invention
The present invention relates to an imaging apparatus and an exposure control method executing a light-metering operation by using an accumulation-type image sensor.
2. Description of the Related Art
An imaging apparatus such as a digital camera or a digital video camera generally requires a light-metering range of approximately −5 to +15, as Bv values in additive system of photographic exposure (APEX) units. Namely, such imaging apparatus has a light-metering dynamic range of approximately 20 levels. However, when an accumulation-type image sensor executes a light-metering operation, the dynamic range in a single accumulation operation is less than 20 levels.
To address this problem, there is a known technique of generating an image having a dynamic range greater than the dynamic range obtained in a single accumulation operation (imaging). Based on this technique, an identical scene is captured by an accumulation-type image sensor a plurality of times with different exposure levels, and obtained image signals are compounded. This technique is generally referred to as high dynamic range (HDR) imaging.
Similarly, various documents are proposing increasing the dynamic range of a light-metering apparatus using an accumulation-type light-sensitive element. For example, Japanese Patent Application Laid-Open No. 6-130462 discusses a light-metering apparatus using an accumulation-type light-sensitive element. When executing a light-metering operation, this apparatus alternately uses a prolonged charge accumulation time and a shortened charge accumulation time. In this way, even when the luminance difference in an object field is very large, the apparatus can obtain light-metering values from low to high luminance parts.
In addition, imaging apparatuses having an object tracking function are conventionally known. This function recognizes a main object within an imaging screen and automatically tracks the position of the main object. Use of this object tracking function continuously enables automatic focus adjustment control (AF control) or exposure control (AE control) on a moving main object.
Examples of an imaging apparatus having such object tracking function include an imaging apparatus having, other than an image sensor, a light-metering unit used for a light-metering operation, such as a single-lens reflex camera. Japanese Patent Application Laid-Open No. 7-110429 discusses an apparatus that tracks an object, based on light-metering data supplied from a light-metering unit.
In addition, Japanese Patent Application Laid-Open No. 2011-35765 discusses a tracking technique. Based on this technique, a feature region of a human, for example, is extracted from an image signal acquired from a light-measuring unit, and a correlated image is determined within subsequently acquired images. In this way, the extracted feature region can be focused and utilized for exposure control.
However, if it is attempted to increase the operable light-metering range based on the technique discussed in Japanese Patent Application Laid-Open No. 6-130462, with the technique of tracking an object based on light-metering data supplied from a light-metering unit as in the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 7-110429, the following problems occur.
If a light-metering operation using a prolonged charge accumulation time and a light-metering operation using a shortened charge accumulation time are alternately executed to increase the operable light-metering range, each of the luminance ranges in which a light-metering operation is possible is different from each other. Thus, even if the luminance of a target tracking region is within the operable light-metering range in one light-metering operation, the luminance may be outside the operable light-metering range in another light-metering operation. If calculation is executed for correlation between such two light-metering data to track an object, accuracy of the correlation calculation is decreased. As a result, the object cannot be tracked or accuracy in tracking the object is decreased.
Similarly, if the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 2011-35765 alternately executes a light-metering operation using a prolonged charge accumulation time and a light-metering operation using a shortened charge accumulation time, a feature region may or may not be extracted. Thus, a feature region cannot be stably extracted.